1. Field of the Invention
The present invention concerns a method for producing a ceramic sandwich type honeycomb-shaped structure, especially for large lightened mirrors, as well as the ceramic structure obtained by said method.
2. Description of Background Information
A large number of telescopes have been sent into space and carried by satellites for extremely varied applications. These telescopes have a solid glass mirror so as to withstand the stresses of space, basically mechanical and thermal stresses, while providing the mirror portion with the best possible optical quality so as to obtain extremely high performance.
Glass is well-known for:
its dimensional stability due to its extremely low expansion coefficient, PA1 its capacity to be cast into a given shape, PA1 its polishing capacity, PA1 its mechanical qualities. PA1 cutting along the desired profile of a preform of a honeycomb-shaped structure whose cell walls have an organic matrix with a fibrous reinforcement while taking into account any possible shrinkages, PA1 pyrolysis of this honeycomb structure preform so as to retain solely the carbon whilst keeping the initial profile, PA1 silicidation of the carbon of the preform so as to obtain a microporous SiC core, PA1 infiltrating type reinforcement by SiC chemical vapor deposition of the cell walls of the honeycomb structure, PA1 formulation of a felt type sheet, assembling the sheet on a one face of the core, thus covering the cells and densification of this sheet so as to obtain SiC skins on the surface of the core so as to form a SiC structure. PA1 placing a tufted felt with short ex-raxon carbon fibers on each of the faces of the core, PA1 infiltrating type stiffening and linking of said felt to the cell walls of the cells by means of a SiC chemical vapor deposition, PA1 binding by a SiC-based cells so as to reduce porosity on the surface, and PA1 infiltrating type chemical vapor deposition of a thin coating of SiC with a slow kinetics phase for densifying the surface coating, and a thicker coating of about hundreds of u during a second phase with faster kinetics. PA1 making of a SiC felt sheet of whiskers coated with an organic binder with a low voluminal percentage of fibers of about between 5 and 20%, a small quantity of binder between 3 and 15% in weight and a small thickness of about one millimeter, PA1 cutting of portions of this felt sheet of whiskers to the profile of the faces of the core with a reinforced honeycomb structure and placing of these cut portions on these faces with a subsequent piercing to imprint the sheet with the core which constitutes a sandwich, PA1 baking of the sandwich produced so as to polymerize the organic binder and fix this sandwich in the desired shape, PA1 pyrolysis of the sandwich so as to eliminate the organic binder, PA1 chemical vapor deposition of SiC with slow kinetics so as to form the skins on the core and a predensification, and PA1 chemical vapor deposition of SiC on the skins made of predensified SiC whisker fibers so as to mechanically reinforce and seal the sandwich which results in producing a half-finished product. PA1 placing in suspension of SiC whisker fibers in an ethyl alcohol/phenol solution, PA1 flocculation by adding water to the solution with continuous agitation of the mixture, and PA1 filtering flocks of whisker fibers on a fine mesh screen of so as to obtain a sheet of whisker fibers orientated randomly. PA1 reinforcement of the honeycomb core in the areas aligned with the attachment points by introducing carbon fibers, by punch cutting a felt made of "ex-rayon" carbon fibers, into the cells aligned with the attachment points; the walls of the cells being used as a punch. PA1 optionally placing of a receiving part aligned with the insert prior to assembling and densification of the skins, stiffening by SiC chemical vapor depositing aligned with reinforced zones, sealing with a SiC-based paste so as to reduce the porosity of the surface of the felt and ensure bonding of the receiving part, and PA1 assembling and densification of the skins, PA1 machining of the sandwich aligned with the insert for placing of the holding device or for mounting the holding device in the receiving part, PA1 chemical vapor deposition of SiC so as to form a sealed skin on the surface of the felt and coat the peripheral zone of the holding device mounted in the insert with a possible prior sealing with the paste.
Nevertheless, glass does have one drawback which is particularly important for space applications, namely its weight.
Any additional weight requires an additional thrust to be placed into orbit, which leads to significantly increased costs.
Moreover, once in orbit, the weight remains prejudicial as it is necessary to have a power motorization proportional to the weight and it is necessary to combat the focus of inertia, a force which limits accuracy.
In addition, the fragility of glass, especially during mounting but more generally during the various machining phases, is also a serious drawback.
The space industry and connected industries which use the same techniques, especially the aeronautics industry, are seeking a structure which possesses good mechanical characteristics, especially high rigidity, as well as an extremely low coefficient of expansion and being as light as possible.
Moreover, for certain applications, it is essential that the material resist oxidation at extremely high temperatures of about 1600.degree. C. without its mechanical qualities being significantly altered at these temperatures.
Silicon carbide, SiC, is one of several high performance materials having properties compatible with these strict and highly specific specifications since its density is low, d=3.25, its longitudinal elasticity module is high, E=600 Gpa, its coefficient of thermal expansion is low .alpha.=4.10.sup.-6.degree. C..sup.-1, and its coefficient of thermal conduction is sufficiently high for the applications envisaged, .lambda.=200 W/m..degree. K.
Moreover, SiC is insensitive to radiations emitted in a synchrotron, a cyclotron or in high power lasers by virtue of its low atomic number, although it can be used in these applications as a deviation mirror.
Other applications may be possible with this SiC-based ceramic. The method includes applications involving not only extremely lightened mirrors with high dimensional stability, but also thermal screens for aircraft, aircraft fire protective screens, thermic exchangers or catalyst supports.
It is also known that in cryogenic optical systems, the thermic mass effect is reduced by the use of SiC with the result of facilitating cooling of the mirror and maintaining the right temperature.
A large number of techniques have been developed to produce SiC structures, but a large number of difficulties limit the possibilities for lightening the products manufactured. The size and shape are also subjected to production stresses.
WO 88/07688 concerns a lightened mirror made from SiC and a method for producing this mirror.
This particular mirror includes an SiC foam preform made from polyurethane foam cut to the sought-after profile and from which all the heteroatoms are eliminated, except carbon. SiC is then deposited on this preform by any suitable method so as to obtain a monolithic porous block of SiC which is densified by depositing a fine coat of SiC on the surface so as to provide the surface with good adhesion capacity.
The block obtained is machined so as to provide it with surface evenness or the desired curve. The machined faces receive a thin SiC coating by means of chemical vapor deposition. This coating reinforces the mechanical resistance of the block and which may be polished so as to provide the surface with optical qualities.
This block nevertheless exhibits inadequate resistance for certain applications in which the mechanical stresses are high and the dimensions are reduced, with the result that the strength, especially mechanical resistance, of a SiC block with a foam structure is too weak. It is also possible that the penetration of SiC into a thick foam block poses problems.